Alles "will be leaving shortly," White House press secretary Sarah Huckabee Sanders says, adding that President Donald Trump "has selected James M. Murray, a career member of the USSS, to take over as director beginning in May."

NBC's Pete Williams reports that the firing was "not based on any single precipitating event."

U.S. Secret Service Director Randolph Alles is soon to be out of his job.

White House press secretary Sarah Huckabee Sanders said in a statement that Alles "has done a great job at the agency over the last two years, and the President is thankful for his over 40 years of service to the country."

Alles, 65, "will be leaving shortly," Sanders said, adding that President Donald Trump "has selected James M. Murray, a career member of the USSS, to take over as director beginning in May."

An initial report from CNN cited multiple administration officials saying Trump had instructed his acting chief of staff, Mick Mulvaney, to fire Alles. A source, who asked to remain anonymous to discuss Alles' departure, told CNBC that the Secret Service chief had been asked to submit an exit plan 10 days ago.

In a message to the Secret Service workforce Monday, Alles insisted that he had not been fired. Rather, Alles wrote that he was "told weeks ago by the administration that transitions in leadership should be expected across the Department of Homeland Security":

"No doubt you have seen media reports regarding my 'firing.' I assure you that this is not the case, and in fact was told weeks ago by the Administration that transitions in leadership should be expected across the Department of Homeland Security. The President has directed an orderly transition in leadership for this agency and I intend to abide by that direction. It is my sincere regret that I was not able to address the workforce prior to this announcement.

"It has been my great honor to serve as Director of the U.S. Secret Service. I want to personally thank you all for a job well done. Your pride, strength and resilience is what has, and will continue to, allow this agency to accomplish great things. Your dedication and sacrifice continue to make the U.S. Secret Service an elite law enforcement agency; one that will remain so well into the future.

"Assistant Director of the Office of Protective Operations James Murray has been named the incoming Director of the Secret Service. Jim is a consummate professional, a true leader and I have great confidence in his capabilities. Please give him the outstanding support that you have shown me these past two years."

The staffing development leaves the Department of Homeland Security without a Senate-confirmed official in yet another top role.

Trump tweeted Sunday that Kevin McAleenan, the current commissioner of Customs and Border Protection, will serve as acting Homeland Security secretary.

A White House official, who asked to remain anonymous to discuss Alles' departure, told CNBC that the president has been frustrated with the increasingly high numbers of monthly illegal border crossings.

"The president deserves people in positions who will carry out his agenda," the White House official told CNBC.

Less than a week earlier, federal authorities charged Chinese national Yujing Zhang with illegally gaining entry to Trump's Mar-a-Lago resort in Palm Beach, Florida, slipping past at least five Secret Service agents and making it all the way to the main reception area of the compound.

NBC News' Pete Williams reported Monday that, according to an administration official, Alles' firing was "not based on any single precipitating event." The decision was made 10 to 14 days ago, the official told NBC — before the Mar-a-Lago intrusion.

Trump had praised the Secret Service in the wake of the incident. Asked at the time about his confidence in the luxury club's security, Trump said, "I could not be happier with Secret Service. Secret Service has done a fantastic job from day one. Very happy with them."

Nielsen tweeted that she will remain in her role until Wednesday to allow for a smooth transition. At that time, DHS won't have a Senate-confirmed commissioner, either.

White House officials told CNBC that they expect Claire Grady, the current DHS undersecretary for management, will also be leaving in order to clear the way for the president's pick to take over the department.